Self-oscillating additive mixer circuit



March 15, 1960 Filed July 15, 1957 NEGATWE FEEDBACK AT INTERMEDIATE FREQUENCY \NEGATNE FEEDBACK AT P UNDESIRED BLOCKlNG FREQUENCY INVENTOR HE IN 2 306 K AGEN;

2,928,936 SELF-OSCILLATING ADDITIVE MIXER CIRCUIT Heinz hock, Hamburg, Germany, assignor .to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Application July 15, 1957, Serial No. 671,748 Claims priority, application Germany August 24, 1956 4 Claims. (Cl. 250) Self-oscillating additive mixer circuits are known using a triode, the output circuit of which includes, between the positive terminal of the supply source and the anode, the series combination of a feedback resistor and the primary inductance of an intermediate-frequency output filter. The end of the resistor more remote from the supply source is connected to the tube grid, which is provided with a comparatively large leak resistor, the intermediate frequency oscillations being fed back through this connection.

Usually, the value of the said feedback resistor is about 10 thousand ohms. Obviously, this resistor involves a considerable direct voltage drop, which, under certain circumstances, reduces the performance of the tube and is particularly inadmissible if use is made of a valve which is meant to be operated at a low anode voltage and is applied from a suitable battery or the like.

It is an object of the present invention to provide a solution of the problem of avoiding this supply voltage loss without impairing the effect of the feedback resistor.

According to the invention, this is ensured in a circuit arrangement ofthe above mentioned kind by including in v duced across it also prevents the oscillator from overoscillating, that is to say, from producing comparatively low-frequency relaxation oscillations, which occur especially if the grid leak resistor is large, for example is of the order of one megohm and of the grid time constant, which is determined by the grid leak resistor and the capacitances connected in parallel therewith, is comparatively large. These relaxationroscillations (over oscillations) provide a high degree of modulation of the useful oscillations and are particularly troublesome if their frequency falls within the. frequency range of the signal being transmitted.

Hence, it is necessary for the anode lead to include also a resistor which provides a low-frequency feedback voltage. It has however, been found that the value of this resistor can be materially smaller than that of the resistor hitherto used, which provides the intermediate frequency feedback voltage also, so that in actual fact a material reduction of the direct-voltage loss is obtained in the circuit arrangement in accordance with the invention without altering its operation.

in order that the invention may readily be carried into effect, one embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawing.

2,928,936 Patented Mar. 15, 1960 P CC Referring now to the drawing, ultrahigh-frequency oscillations are supplied from a dipole antenna -1 through a lead 2 to an antenna coupling coil 3. This coil is coupled to an input coil 4, one end of which is connected to ground through a tuning capacitor 5, the other end being connected to ground through a feedback capacitor 6 which will be described more fully hereinaften The center of the input coil 4 is connected, through a coupling capacitor 7, to the center of a grid coil 8, one end of which is connected to the control grid of a triode 9, the other end being connected to ground through 'a grid leak resistor 10. In parallel with this grid leak resistor10, there is connected a trimming capacitor 11 which is 'adjusted to about the value of the grid-cathode capacitance of the tube 9, so thatthe tapping on the coil 8, to which the coupling capacitor 7 is connected, is free from the self-oscillations of local frequency. These self-oscillations are'generated in a parallel resonant circuit comprising an inductance 12 and a tuning capacitor 13, one end of this circuit being connected to ground, whilst the other end is connected to the anode of the triode 9 through a connecting capacitor 14. The cathode of the triode 9 is connected to ground. The anode circuit of the 'tube9 includes the primary inductance -15 ofan intermediate frequency output bandpass filter. By means of the connecting capacitor 14 and the stray capacitances, the primary inductance 15 is tuned to about the intermediate frequency and furthermore is coupled to the inductance of a resonant circuit 16,17 so as to produce a bandpass filter circuit from which the intermediate-frequency oscillations can be taken. v

In the known circuit arrangement, the terminal of the inductance 15 more remote from the anode of the tube 9 is connected, through a resistor of about 10 to 30 thousand ohms to the positive terminal of the supply source and also to the junction of the feedback capacitor 6 and the input coil 4. The feedback capacitor 6 andthe coupling capacitor Tom the one'hand and the grid-anode and anode cathode capacitance of the valve 9 on the other hand together constitute a bridge circuit which, similarly to a Colpitts circuit, provide feedback of the oscillations produced 'at the intermediate-frequency output impedance by means of capacitive voltage division.

In order to avoid the inconvenient voltage drop across so large a resistor in the anode circuit without affecting the intermediate-frequency feedback, a choke coil 18, which has a comparatively large inductance for intermediate-frequency oscillations, is connected between the end of the output inductance 15 connected to the capacitor 6 and the positive terminal of the voltage supply source.

In order to ensure a small grid current so as to reduce the noise and in order to avoid the occurrence of inconvenient harmonics, the grid leak resistor 10 is made comparatively large. If, however, no further steps are taken, so-called over-oscillations would be produced, that is to say intermittent oscillator oscillations, owing to the fact that, due to grid rectification, the grid capacitance, more particularly the coupling capacitor 7, is suddenly charged to a high voltage and subsequently is discharged at a low rate.

Hence, in the circuit arrangement in accordance with the invention, there must be provided, in addition to the choke coil 18, a feedback resistor 19 which is connected between the choke coil 18 and the supply source. If, now, relaxation oscillations occur, which give rise to a corresponding fluctuation of the anode current, they produce, across this resistor, a negative-feedback voltage which, similarly to the intermediate-frequency feedback voltage, is supplied through the connecting lead to the capacitor 6 and through the coupling capacitor 7 to the grid of the valve 9.

Preferably, the value of the negative feedback resistor 19 is at least about equal to the reciprocal of the mutual conductance of the tube 9 and preferably is several times as large. Thus, with a mutual conductance of from 2 to it) milli-amperes per volt, a resistor 19 of from 100 to 500 ohms is sufi'icient; it will be appreciated that this value is materially less than the value of the conventional resistor, from which the intermediate-frequency feedback voltage was also taken.

Thus, the circuit arrangement in accordance with the invention ensures a material reduction of the direct voltage drop without modifying its operation.

Especially, if at the intermediate frequency, the inductive resistance of the choke coil, 18 is about equal to the value of the negative feedback resistor 19, it is of advantage to provide, between the junction of the choke 18 and the, negative-feedback resistor 19 and earth, a capacitor 20 which offers a comparatively lowimpe'dance to the intermediate-frequency oscillations, for then the impedance between the junction of the inductance 15 and the choke 18 and ground or supply source is effectively in parallel with the inductance 15 through the series combination of the anode-ground capacitance of the tube 9. Thus, the choke 18 is effectively in parallel with the inductance 15. If the ratio between the inductive resistance of the choke 18 and the resistor 19 connected in series therewith and hence the Q-factor of this branch is small, the intermediate-frequency output filter is heavily I damped; thisis avoided by the inclusion of the capaci tor 20.

In order to ensure a satisfactory negative feedback of the. above-mentioned relaxation oscillations, the frequency of which is comparatively low as compared with the input frequency, the oscillator frequency and generally the intermediate frequency also, it is of advantage for the time constant of the RC-section comprising the negative feedback resistor 19 and the overall capacitance connected in parallel therewith to be smaller than, preferably to be small as compared with, the time constant of the RC-section comprising the grid leak resistor v10 of the valve 9 and the overall capacitance connected in parallel therewith. I

As has been stated hcreinbefore, the choke 18 is el'I'ec tively in parallel with the inductance 15, and the value of the choke 18 must be large as compared with the inductance 15. For the sake of completeness, it should be noted that the effective value of the choke 18 is multiplied by a factor ada:

where C is the capacitance of the feedback capacitor 6 increased by any further capacitances which are connected in parallel with respect to the intermediate frequency (particularly the tuning capacitor 5) and C represents the capacitance between the anode and cathode of the valve 9, particularly the connecting capacitor 14.

It should be avoided that, at the intermediate frequency, the choke 18 comes into parallel resonance with the capacitor 6 (and with any further parallel capacitances), and in particular ..the impedance between the feedback lead (from the choke 18 to the feedback capacitor 6) and earth must be capacitive in order to maintain 5 the abovementioned capacitive bridge circuit. Conse quently, the said parallel resonance frequency must be lower than the intermediate frequency, and preferably must be equal to or lower than about one half of the iintermediate frequency.

What is claimed is: p

l. 'A self oscillating additive mixer circuit comprising an amplifier device having control grid and output electrodes, an oscillatory circuit connected to said device to cause self oscillations, 'a source of input oscillations, input circuit means connected between saidgrid and source of input oscillations whereby'said self oscillations and input oscillations are mixed in said device to provide inter- -mediate frequency'oscillations, a source of operating voltage, an output impedance member having one end con- 20 nected to said output electrode, a resistor having one end connected to said source of operating voltage, an inductor connected between the other ends of said resistor and impedance member, said input circuit means including grid leak bias resistance means, an intermediate frequency feedback connection between the junction of said inductor and impedance member and said input circuit means, and a capacitor connected between ground and the junction of said inductor and resistor means, said capacitor having low impedance to said intermediate frequency oscillations, the time constant of said resistor and capacitor being small in relation to the time constant of said grid leak resistance means and effective circuit capacities in parallel therewith.

2. A circuit as claimed in claim 1, in which the inductance of said inductor is larger than that of said output impedance member. Y Y

3. A circuit as claimed in claim 1; in which said feedback connection comprises a feedback capacitor connected between said input circuit and electrical ground,

40 the combination of said inductor and said feedback capacitor having a resonance frequency which is lower than said intermediate frequency;

4. A circuit as claimed in claim 1, in which said amplifier device has a value of mutual conductance, the value of said resistor being equal to'at least the value of the reciprocal of said value of mutual conductance.

ReferencesCited in the file of this patent UNITED S TATEs PATENTS 2,713,634- Beers et al. V July 19, 1955 2,789,213 Marks ct a1. Apr. 16, 1957 a a a I "FOREIGN PATENTS 1,098,772 Fra ce Mar. 9, 1955 OTHER REFERENCES Theory and Applications of Electron Tubes, by Reich, 2nd edition, published by 'McGraw-Hill, Inc., 1944, pp. 2.10 .and l84.

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